1. Field of the Invention
The present invention relates generally to a liquid crystal display device and, more particularly, to an improvement pertaining to a pillar-shaped spacer for keeping constant a distance between two substrates for sealing a liquid crystal in the liquid crystal display device.
2. Description of the Background Art
A liquid crystal display device generally employed nowadays is constructed such that two glass substrates having electrodes are set in a face-to-face relationship, peripheries of these two substrates exclusive of a liquid filling port are fixed by a bonding agent, a liquid crystal is interposed between the two substrates, and the liquid crystal filling port is sealed by a sealing agent. Plastic beads or the like having a uniform particle diameter are dispersed between the substrates by way of a spacer for keeping a fixed distance between these two substrates.
A liquid crystal display device for color display includes color filters R, G and B of color layers that are disposed on one of the two glass substrates. For instance, a color dot matrix liquid crystal display device based on a simple matrix drive includes a Y-substrate having a Y-electrode subjected to band-like patterning in a lateral (Y) direction and an X-substrate having color layered under the X-electrode subjected to the band-like patterning in a vertical (X) direction, wherein the X- and Y-substrates are provided in a face-to-face relationship so that the X- and Y-electrodes are substantially orthogonal to each other, and a liquid crystal material is sealed in therebetween. The liquid crystal display device may involve the use of display systems such as, e.g., TN (Twisted Nematic) type, STN (Super Twisted Nematic) type, GH (Guest Host) type, or ECB (Electrically Controlled Birefringence) type and a dielectric liquid crystal. The sealing agent involves the use of, e.g., a bonding agent of a heat hardening type or an ultraviolet ray hardening acrylic or epoxy group. Further, the color active matrix drive liquid crystal display device is constructed of a TFT active matrix substrate, i.e., an active matrix substrate formed with a switching element, e.g.; a thin-film transistor (TFT) with a semiconductor layer composed of amorphous silicon (a-Si), and a pixel electrode, a signal electrode and a gate electrode that are connected thereto, and also an opposite substrate disposed in the face-to-face relationship with the TFT active matrix substrate. The color filters R, G and B are provided on the opposite substrate. Disposed on a screen peripheral portion is a silver paste serving as an electrode transfer member (transfer) for applying a voltage to the opposite substrate from above the active matrix substrate. The two substrates are electrically connected by this electrode transfer member, and the liquid crystal material is sealed in between those two substrates. Furthermore, polarizing plates are secured on both side of those two substrates.
In the liquid crystal display device using the plastic beads as a spacer, however, orientations of the liquid crystal peripheral to the spacer, which liquid crystal is scattered between the two substrates, are disordered, resulting in such a tendency that the contrast declines due to a leakage of light beams from the spacer peripheral portion. Moreover, the spacers are hard to disperse uniformly and are arranged with ununiformity during a step of dispersing the spacers on the substrate. This results in a display defect, which in turn brings about a decreases in yield of the product.
Moreover, in the liquid crystal display device of nowadays, when the two substrates are bonded in the forming step, there is taken a method of coating a sealing material over the one-side substrate of the two substrate, superposing this substrate on the other substrate, and hardening the sealing material while applying a pressure by a pressing device. On this occasion, a strong pressure tends to be applied upon an outer peripheral portion of the substrate, and therefore a substrate-to-substrate gap of the outer peripheral portion is easy to reduce. This might cause a display defect attributed to a deterioration in terms of cell gap.
On the other hand, if a spacer density on the entire surface of the substrate is increased as a countermeasure against the deterioration of the cell gap, there arises a problem in which vacuum bubbles are easy to produce in a liquid crystal layer.
Proposed in, e.g., Japanese Patent Application No. 7-212192 by way of a countermeasure against the above-mentioned are a method of forming the spacers in positions exclusive of a display area while superposing color layers of a color filter, and a method of forming pillar-shaped spacers by use of photo resists.
There were, however, found two points in the pillar-shaped spacer, which should be improved.
A first point is concerned with a rubbing process. In the case of depositing an orientation film after forming the pillar-shaped spacer and forming a multiplicity of hyperfine grooves in the whole orientation film in one direction by use of a rubbing cloth, the rubbing cloth contacts the pillar-shaped spacer. On this occasion, a comparatively large stress (frictional resistance) is applied upon the rubbing cloth because of the pillar-shaped spacer taking a square or round shape, with the result that staples of the rubbing cloth are bent or damaged. If the rubbing cloth containing defective staples is continuously used, this leads to an execution of an ununiform rubbing process, which in turn might cause a display defect.
Further, the staples of the rubbing cloth are temporarily bent by the pillar-shaped spacer, and consequently the ununiform hyperfine grooves are partially formed till the bent staples of the rubbing cloth revert to the previous state. This conduces to a partial rubbing deterioration in the vicinity of the pillar-shaped spacer, which might further cause the display defect.
A second point is concerned with a mechanical strength of the pillar-shaped spacer. The pillar-shaped spacer is formed directly on the substrate by using a resin or a photosensitive resin. The resin and the photosensitive resin are high polymer materials and therefore insufficient in terms of the mechanical strength such as a rigidity and an adhesion. The spacer is easy to exfoliate ad deform. consequently, there are produced such an inconvenience that a reliability on the liquid crystal display device decreases.
Further, an inter substrate distance is as narrow as 2 xcexcm, and the pillar-shaped spacer might become an obstacle against an inflow of the liquid crystal when the liquid crystal permeates. Especially a ferroelectric liquid crystal is hard to permeate.
Moreover, as explained above, in the liquid crystal display device, the liquid crystal is sealed in between two pieces of upper and lower substrates, and the display is performed by applying the voltage across the electrodes of the upper and lower substrates and controlling motions of liquid crystal molecules. However, the voltage is applied across the electrodes of the upper and lower substrates, and hence a power supply is hitherto connected to only one substrate. Then, silver paste as a transfer is disposed at a screen peripheral portion by a dispenser or the like, and the two substrates are electrically connected by this transfer.
In the above-mentioned transfer forming step, however, large costs for equipments and expenditures for labors are needed. Further, it is required that a portion for disposing the transfer in the screen peripheral portion be provided to have a considerable areal size (over approximately 1-2 mm) in a non-display area peripheral to the liquid crystal cell, including a margin for the transfer forming step. Therefore, the non-display area peripheral to the liquid crystal cell can not b e reduced. Moreover, the liquid crystal material and the orientation film are contaminated with impurities mixed when filling the dispense r with the transfer material, resulting in the display defect and a drop in terms of the yield.
It is a primary object of the present invention to provide a liquid crystal display device capable of reducing a display defect attributed to a deterioration in cell gap, increasing a yield, requiring a small amount of costs and exhibiting a high display performance.
It is another object of the present invention to provide a liquid crystal display device capable of preventing a disorder in orientation that is caused in a rubbing process upon an introduction of a pillar-shaped spacer, and thereby preventing the display performance from be decreased.
It is still another object of the present invention to provide a liquid crystal display device capable of facilitating permeation of a liquid crystal.
According to the present invention, a density or volume os the spacers is set higher in an off-display area than in a display area.
Further, according to the present invention, the number of steps can be decreased by using the spacer itself as a transfer.
Moreover, according to the present invention, the contact with the rubbing cloth when rubbed is reduced, and hence the spacer assumes a configuration with a small width. With this configuration, the mechanical strength of the spacer can be increased, and both the deterioration of the rubbing cloth and the defect in the orientation can be reduced.
Further, the contrivance is given in terms of the spacer position so as not to exert an influence of the orientation defect caused by the rubbing upon the pixel or so as not to make the influence occur conspicuously. It is therefore possible to prevent a deterioration in display quality.
The spacers may be disposed along a permeation flow of the liquid crystal. With this arrangement, the resistance is reduced, thereby making it possible to facilitating the permeation of the liquid crystal while keeping the mechanical strength of the spacers.
Additionally, a distribution of the spacers has an optimal value, and the distance between the two substrates can be uniformly precisely controlled within the screen by prescribing the optimal value.